13:27 And those who disbelieve say: “Why is not a Sign from his Rabb sent down upon him?” You say: “Allah leaves in straying whom He wishes, and He guides towards Himself one who turns.”

13:27 i.e. why did Muhammad not came with a ‘sign’ as a testimony of his mission. Apparently the skeptics wanted to see some supernatural wizardry or astonishing event to convince their heart (2:118; 6:109,158; 10:20; 17:90-93; 21:5; 29:50; 40:78). The Prophet never claimed he had power to show miracles, and the Qur’ān asserted that it was not his mission to show miracles in proof of his prophethood (17:90-93; 29:50). Had manifestation of astonishing incident been the tool to convince skeptics, Allāh would not have repeatedly asked to ponder over the orderliness in the creation and to exercise one’s rational thought process. It is not the intention of Allāh to mesmerize mankind with dumbfounded wizardry to convey His message and to prove His existence. Had Allāh adopted such a strategy, it would have amounted to coercing mankind to believe through use of external instruments rather than using the inner strength of the divine message (cf. v. 7). Since mankind is given freedom of choice in the matter of faith, he could not be tempted towards religion or cleverly driven towards it by means of outlandish ‘signs’. Since people choose to disbelieve on the basis of their freedom of choice, Allāh lets those who disbelieve remain in straying.